Many patents deal with the dehydrogenation of long chain-paraffins. They generally provide means for limiting side reactions that focus on the process and/or on the catalytic formulation.
Thus, the great majority of the patents that deal with long chain paraffin dehydrogenation are concerned with the addition of hydrogen to the feed for H2/hydrocarbons molar ratios ranging between 1 and 10 (EP-0,462,094 B1). The purpose of this make-up hydrogen is to limit or to retard coke formation at the catalyst surface without too negative an effect on the conversion of the n-paraffins being observed. In patent EP-0,462,094 B1, the choice has been made of working at a lower temperature (<450° C.) and with lower H2/HC molar ratios ranging between 0.5 and 1.9. Another solution provided by U.S. Pat. No. 3,448,165, U.S. Pat. No. 3,907,921 and U.S. Pat. No. 5,233,118 consists in injecting a small amount of water and/or of sulfur with the hydrocarbon feed to be dehydrogenated. The water can be injected at constant flow rate or at a flow rate that increases gradually with the operating time of the catalyst. It has been reported that optimum performance was obtained by increasing the injection of water with the temperature of the reactor during the working cycle.
As regards the formulation, U.S. Pat. No. 4,716,143 discloses a catalyst based on supported platinum such that the platinum distribution is limited to the outer surface of the support over a maximum thickness of 400 μm. The advantage of such a choice lies in that a distribution over the periphery of the support decreases parasitic reactions and, consequently, improves the catalyst performances. However, this type of distribution only rarely allows one to obtain homogeneous platinum/modifier atomic ratios on the particle scale (nanometer). Examples of the most commonly used platinum modifiers are group IIIA and group IVA elements, in particular tin (U.S. Pat. No. 3,745,112). The role of the tin predominantly present at the catalyst surface in an oxidation state of +2 and/or + 4 is to modify the properties of the Pt particles, which allows to increase the catalyst selectivity and stability (by hydrogenolysis reduction for example). Tin also has an impact on the acidic properties of the support. Another example of a platinum modifier is indium, notably mentioned in U.S. Pat. No. 4,551,574, EP-B-183,861 and JP-B-91,041,211. In fact, the latter confers better stability while also inhibiting the side reactions of deep dehydrogenation (polyolefins) and of skeleton isomerization (branched hydrocarbons).
In order to limit side reactions, it is interesting to deposit an alkali or alkaline-earth metal whose role consists in taking part in the neutralization of the acid sites of the support of low and medium strength. Thus, even limited addition of lithium (0.1 wt. %) allows to neutralize these acid sites responsible for the formation of isomerized and light products (cracking reactions). Aromatics formation can also be decreased through lithium addition. However, it is also known that this addition leads to a decrease in the total activity of the catalyst. This decrease is often linked with a phenomenon of metallic phase coverage by the alkali metal.
As for preparation, U.S. Pat. No. 5,482,910 describes a catalyst preparation method that involves using a chelating ligand such as EDTA. An aqueous solution comprising the chelating ligand and a salt of an alkali or alkaline-earth metal is first prepared at a temperature ranging from about 80° C. to the boiling point temperature thereof. This solution is then mixed with a solution comprising a group VIII metal. This solution is aged at a temperature ranging between about 40° C. and 100° C., then it is used for impregnating the support. The impregnated support is thereafter dried, calcined, optionally oxychlorinated and finally reduced.
U.S. Pat. No. 3,259,589 describes a catalyst preparation method comprising preferably platinum preferably deposited on an alumina. The platinum can be introduced in complexed form by an organic acid of HOOC[CRR′]n COOH type, with R=OH, H or an alkyl function and R′=H, an alkyl function or COOH.
U.S. Pat. No. 2,889,287 describes a hydrocracking catalyst preparation method comprising at least one active metal selected from the group consisting of groups VI and VIII metals of the periodic table and the oxides of said metals, a support selected from the group consisting of alumina and silica-alumina. The active metal(s) is (are) deposited on the support by impregnation of the support with a complex of the metal(s), then the impregnated support is dried and calcined.